OK, even if you're careful and go slow, you can still experience a bit of tearing when you're having anal sex, especially if your body isn't used to it. And it's also common not to even notice it because it's so minor and there are so many sensations happening at once. And, given the amount of blood vessels that are down there, even one small tear can leave a disconcerting amount of blood.

So, after having an admittedly awkward but really great first time, Steve gently pulls out of Bucky and goes to reach for the tissues to clean them up...and then promptly freaks the fuck out when he sees the streaks of blood.

Bucky, of course, is confused because, What, no, what are you talking about, you didn't hurt me, why would I have told you stop?